Vertical shaft impact crushers (VSI-crushers) are used in many applications for crushing hard material like rocks, ore etc. U.S. Pat. No. 3,154,259 describes a VSI-crusher comprising a housing and a horizontal rotor located inside the housing. Material that is to be crushed is fed into the rotor via an opening in the top thereof. With the aid of centrifugal force the rotating rotor ejects the material against the wall of the housing. On impact with the wall the material is crushed to a desired size. The housing wall could be provided with anvils or have a bed of retained material against which the accelerated material is crushed.
The rotor of a VSI-crusher usually has a horizontal upper disc and a horizontal lower disc. The upper and lower discs are connected with a vertical rotor wall. The upper disc has an aperture for feeding material into the rotor. The material lands on the lower disc and is then thrown out of the rotor via openings in the rotor wall. The vertical rotor walls are provided with wear tips of a hard material, such as a hard metal or a ceramic, to protect them from wear caused by the material leaving the rotor at a high speed. The wear tips are usually made from a hard material to resist wear. The hard material is however sensitive to impact by large objects, such as stones. Thus rotors are usually provided with means for building a bed of retained material against the vertical rotor wall. The bed of material is intended to protect the vertical wall from wear and to protect the wear tip from impact of large objects.
U.S. Pat. No. 3,970,257 to MacDonald describes a vertical shaft impact crusher having a rotor. The rotor is provided with outflow openings. Each outflow opening is provided with a tip which is held by a replaceable tip plate. The tip plate is mounted on a first segment of a vertical plate, said first segment being substantially tangential to the rotor and located at its periphery. A second segment of the vertical plate is welded to the first segment and extends, with an angle of about 120° to the first segment, from the periphery of the rotor towards a point located at a distance from the centre of the rotor. A build up of material along the vertical plate will protect the vertical plate from wear during operation. It has been found, however, that the build up of material at the vertical plate is not always stable and that the build up in many applications may be eliminated during crusher operation. The result is that the vertical plate, the tip plate and the tip is exposed to wear and impact by the feed material.